Time ram A time ram was what happened if two TARDIS es attempted to occupy exactly the same time and space . The resulting paradox would destroy both TARDISes and cause extreme spacio - temporal destruction in its wake . The fate for those caught in a time ram was what the Third Doctor considered tantamount to " extinction " and " utter annihilation " . ( TV : " The Time Monster " ) As and Krasis departed for Atlantis , the Third Doctor made an attempt to materialise his TARDIS inside of the Master ' s TARDIS with precise control over the frequency and time setting on his own TARDIS . He feared an imprecise measurement would make him place the time setting too low and miss the Master ' s TARDIS entirely . The other danger was setting the time too high , which would cause a time ram . He explained the lethal repercussions to Jo Grant , warning her that she had boarded the TARDIS at her own risk on this trip , after she had insisted to come along with him . The Doctor successfully materialised his TARDIS inside of the Master ' s , but accidentally allowed the Master ' s TARDIS to end up inside of his as well , causing both TARDISes to be snared inside each other . This standoff was eventually broken by force . When the Master trapped Jo inside his TARDIS as Atlantis fell apart while Kronos rampaged , the Doctor threatened to send the Master into time ram — even though it would kill them both and Jo — to prevent him escaping with the Crystal of Kronos , which would allow him to dominate space and time forever . The Master called his bluff , realising he wouldn ' t kill Jo . However , Jo used the controls herself to cause a time ram . They were all saved by Kronos , who brought them into its dimension . ( TV : " The Time Monster " ) When Magnus Greel ' s time cabinet was being drawn out of its zygma beam between the years 5000 and 1872 to materialise in 1935 , the Fourth Doctor and Romana I risked triggering a Time Ram to prevent the creation of an alternate timeline and ensure Greel arrived in the past to be defeated in London in 1889 ; by materialising the TARDIS where the Time Cabinet was trying to arrive , they forced the Cabinet to return to its original path as the location it was trying to arrive at was already occupied . ( PROSE : " The Shadow of Weng - Chiang " ) According to the Fifth Doctor , a time ram caused by two TARDISes materialising in the same place would cause a black hole large enough to destroy an entire universe . ( TV : " Time Crash " ) It could be narrowly avoided by a TARDIS materialising within another . ( TV : " The Time Monster " , " Logopolis " , " Time Crash " ) The Sixth Doctor performed a time ram against the remains of Iris Wildthyme ' s TARDIS at the centre of a dimensional nexus . Local conditions limited the effects to merely eliminating some possibilities from reality , but the force was still enough to destroy the rest of Iris ' TARDIS . ( AUDIO : " The Wormery " ) The Profane Virus of Rassilon was a theoretical computer virus that would force a TARDIS to Time Ram itself mere nanoseconds in its own past ; the Master infected TOMTIT with nanobots that would infect the Doctor ' s TARDIS with the virus , but Paul Kairos - the mortal identity of Kronos - was able to purge the nanovirus into the higher dimensions and allow the TARDIS to divert its past self . ( PROSE : " The Quantum Archangel " ) The Seventh Doctor attempted to perform a time ram against the Timewyrm in order to destroy her , but she escaped into the Time Vortex . ( PROSE : " " ) The first eight Doctors time rammed their TARDISes against the Master ' s TARDIS in order to defeat him in a pocket dimension where he had framed them by using a conceptual bomb which was erasing the Doctor ' s timeline . ( AUDIO : " The Light at the End " ) When the War Doctor materialised into the Battle TARDIS of Partheus to rescue him , the latter was afraid their TARDISes could time ram and annihilate each other . ( PROSE : " Engines of War " ) Other time - controlling species were also able to utilise time rams . The Dalek s mercilessly used a time ram against a factory default TARDIS and the Eighth Doctor ' s TARDIS when its defences were removed . ( AUDIO : " Tangled Web " , " X and the Daleks " ) Knowing that the repercussions could be catastrophic for both ships , the Tenth Doctor rammed an imitation of the TARDIS in the Time Vortex in an attempt to save Cindy Wu . This caused both ships to crash - land in what the Doctor described as ancient China . ( COMIC : " Sharper Than a Serpent ' s Tooth " )