The Sontaran Stratagem ( TV story ) The Sontaran Stratagem was the fourth episode of the fourth series of " Doctor Who " . It was the first appearance of the Sontaran s in the revived series and saw the reappearance of Martha Jones as a recurring guest character . Also of note was the casting of the Sontaran s . As well as featuring the return of Christopher Ryan in a new role , this episode featured the first " Doctor Who " appearance of Dan Starkey , seen here as Commander Skorr . Primarily recognised later as Commander Strax , a friend of the Doctor , he would go on to appear in various other roles , several of them also Sontaran , in both the TV series and Big Finish Productions ' audio dramas during the years that would follow . Synopsis . Fifty - two people across the world in eleven different time zones die at the exact same time . The only connection : they all have ATMOS installed in their vehicles . Martha Jones , now a UNIT medic , summons the Tenth Doctor back to modern - day Earth to help figure out why , but an old enemy lies in wait . . . Plot . Luke Rattigan and his students toss out reporter Jo Nakashima and her notes from the Rattigan Academy . She protests , saying that she ' s investigating ATMOS , Rattigan ' s invention . It is dangerous . An uninterested Rattigan lets her leave . However , he goes inside and contacts his mysterious employer , recommending termination of Jo . His employer reminds him they don ' t take orders from " humans " , but Rattigan reminds him that he said , " recommend " , and the employer agrees to terminate Jo . Some time later , Jo is in her car ( fitted with ATMOS ) . She leaves a message for Colonel Mace of UNIT , asking him to check out several recent deaths that have happened in ATMOS - equipped cars and to pinpoint the times . At the same time , she follows her ATMOS ' s instructions . After crossing a bridge , the car turns right and stops at the bank of a river . Jo is confused , as she told ATMOS to take her to UNIT Headquarters . She ' s about to find her own way when the car doors lock themselves and the transmission of the car locks itself in the drive position . The car accelerates towards the bank . Jo screams as the car goes flying , landing in the river , where it sinks with her inside . Meanwhile , Donna is flying the TARDIS with mixed results while the Tenth Doctor watches closely ( warning her not to put a dent in the 1980s ) . He gets a call on his mobile phone , from former companion Martha Jones . Martha waits in an alley when the TARDIS materialises . The Doctor introduces Martha and Donna and catches up on where Martha has been since she left the TARDIS . She is now engaged to Tom Milligan and , thanks to the Doctor ' s recommendation , has a job with UNIT as a medical officer . She speaks into a walkie - talkie and gives the all - clear to execute " Operation Blue Sky . " Elsewhere , UNIT forces raid an ATMOS factory and capture the workers . What no one knows is that Luke Rattigan ' s employer is watching the raid on a television monitor and is not very impressed . Martha leads the Doctor and Donna into a lorry containing a mobile UNIT headquarters . She introduces the Doctor to Colonel Mace , who knows about the Time Lord from UNIT files . The Doctor isn ' t impressed either with the overly military " new " UNIT or their handling of civilian workers in the ATMOS factory . Donna compares it to how the United States treats detainees at Guantanamo Bay . Martha and Mace explain why they ' ve executed the raid : just the day before , fifty - two people were simultaneously poisoned to death in their cars across the world . Martha has checked the biopsies and found the unknown toxin had left each of their bodies immediately . The cars in question are of different makes and models but are all fitted with ATMOS . UNIT suspects that the technology is alien in origin . Meanwhile , two UNIT solders , Privates Harris and Gray , are exploring in the lower corridors of the ATMOS factory . Wandering into another corridor , they find two ATMOS workers guarding a door . Rattigan ' s employer remotely orders the two workers to let the soldiers in . Hypnotically , one opens the door . They enter a room with mysterious lighting and a tank with something banging underneath . They figure it must be a cloning tank and someone is growing a body . Gray tries to call for backup , but a squat armoured person appears in the room and praises Harris ' s skills of deduction . He tells Gray that he smells of sweat and fear . After Harris calls him " Humpty Dumpty " , the figure fires a bolt of energy from a staff to disable Harris ' s legs . Gray fires his submachine gun , but it only clicks . The figure reveals that the room is contained within a cordolane signal that excites the copper surface of the bullet and causes expansion in the barrel , rendering their guns useless , as well as blocking their radio signals ( during which time Gray tries using his pistol but finds that is also useless ) . Gray raises the submachine gun to strike a blow and the figure fires at Grey ' s legs , causing him to fall the floor as well . When asked for a name , the figure identifies himself as General Staal of the Tenth Sontaran Battle Fleet , known as " Staal the Undefeated . " He takes off his helmet to reveal a face that horrifies the disabled privates . The Doctor examines the ATMOS equipment and concludes it is advanced but not alien technology . He notes that he didn ' t expect Martha to work for the military , but she points out that he left her and she is working with UNIT on her own terms . Donna has been using her own skills to go over the personnel records . She returns with the Sick Days file , which is empty . There have been no sick employees at the factory in months , if ever , which is more suspicious . The Doctor wants to go see Luke Rattigan , the creator of the ATMOS system . Mace assigns him a private , Ross Jenkins . Meanwhile , Donna and Martha chat and Martha notes that her family suffered because Martha didn ' t tell them about her involvement with the Doctor . In the basement , Staal finishes the hypnotic programming on Gray and Harris , assigns them orders and returns to the Sontarans ' ship via a teleport pod . The Doctor prepares to go the Rattigan Academy and Donna says that she ' s going home . The Doctor thinks she means she is leaving for good . He babbles on about how he ' ll miss her until he figures out she ' s just going home to talk to her mother and granddad . Meanwhile , Martha talks to a factory worker , Trepper , who seems to have come from Poland just to work . She finds him little more than an automaton with a rapid heartbeat , as he speaks in an emotionless monotone and he claims he works 24 hours a day . She goes to find Mace . Gray and Harris offer to escort her to the colonel . They take her to the basement and lock her in the cloning chamber . Donna returns home and explains to Wilfred where she ' s been . He takes it well , but warns her not to tell her mother , Sylvia . When Sylvia comes in and demands to know where Donna has been , Donna says she ' s been travelling . The Doctor and Jenkins travel to the Rattigan Academy , and the Doctor notes that even UNIT jeep s have ATMOS . Jenkins says that until the system has been proven dangerous , the government refuses to remove ATMOS . They arrive at the academy and meet Luke . The Doctor checks out the science labs and goes to Luke ' s reception area , which contains a large piece of tunnel - like sculpture . The Doctor notes that Luke couldn ' t have created the equipment in the science labs . He triggers a control on the piece of " art , " revealing it ' s a teleport pod . The Doctor ends up teleporting the Sontaran spaceship . As they notice him , he teleports back . General Staal makes it through before the Doctor can sabotage the pod . He removes his helmet . The Doctor , realising who he ' s up against , taunts Staal about his honour and bounces a ball off of the wall behind Staal . It hits his probic vent , the Sontarans ' one weakness . Staal , stunned , collapses . The Doctor and Jenkins make their escape . Staal recovers and fixes the teleporter , then returns to his ship with Luke . He has his second - in - command Commander Skorr , " the Bloodbringer " , begin the invasion . Skorr teleports down to the factory and confronts a captured Martha , saying for her , they need something more sophisticated than the hypnotic programming they utilised in the factory drones and the privates . He renders her unconscious and begins transferring her memories into a clone . Staal realises that he is dealing with the Doctor , the last Time Lord . He tracks the Doctor through the ATMOS device in the jeep and drives it into the river . The Doctor overrides the device by reverse psychology : he orders it to do what it ' s doing anyway so it will disregard his orders and do the opposite . It stops just short of the river and the Doctor and Jenkins jump out , expecting an explosion , which turns out to be unneccessary as the ATMOS just sparks and shorts out . With the ATMOS destroyed , Staal assumes that the Doctor is dead and confirms with Skorr that the Martha clone is ready . Luke assures him that the initial test run resulting in the fifty - two dead car drivers was successful and they are ready to unleash ATMOS on a wider scale . The Doctor goes to pick up Donna while Jenkins finds a vehicle without ATMOS so they can get back to UNIT . Wilfred and Sylvia both recognise the Doctor from their previous Christmas encounters with him . The Doctor calls UNIT and talks to the Martha clone . She promises to pass on his warning about the Sontarans but tells Mace that everything is fine . The Doctor then examines the ATMOS in Donna ' s car and triggers a hidden device exposing gas nozzles . The Sontarans detect the activation and Staal realises the Doctor is still alive . He orders the invasion to begin and the Sontaran ships close in . The Doctor realises the gas is poison ous and there are over four hundred million cars on Earth with ATMOS . Wilfred tries to move his car , but at the same time , Staal orders the converters to be activated and every one deadlocked . As Wilfred gets in his car , the doors suddenly lock themselves and the engine starts . As the ATMOS device begins to emit a poisonous gas , the Doctor tries to disable the ATMOS to get Wilfred out . In a supermarket car park , every ATMOS car goes crazy , releasing the same gas while their owners attempt to stop them . At UNIT , soldiers try to destroy their ATMOSes by shooting at them . Mace learns that all other cars equipped with ATMOS have been activated . Meanwhile , the Doctor ' s efforts to get Wilfred out of his car fail . He is interrupted when the gas starts setting off car alarms on the street . He looks up to see other parked cars emitting the gas . Up above , the Sontarans and Rattigan shout a war chant as the invasion begins . Ood < / a > " )