The Invertebrates of Doom ( short story ) The Invertebrates of Doom was the eleventh short story in the Short Trips anthology " " . It was written by Andrew Collins . It featured the Seventh Doctor and Mel . Summary . Eight university students are exploring caves . Deep in a wet part of the cave , they find an artefact with intricate patterns on it . One of the students , Laura Ainsell , touches the artefact . She can ' t remove her hand , and suddenly a shape begins to appear above the artefact . The Seventh Doctor and Mel are tracking " anomalous concentrations of artron energy " that have drawn the TARDIS off course . Their search leads them to a research facility in 1978 England . In his laboratory , Dr Leech and his assistant Ly examine the artefact . Leech is exhausted so Ly brings him some water . Leech thinks his exhaustion comes from the same source that caused the students to be quartered in the medical wing : the artefact . The Doctor and Mel burst in on Leech , and the Doctor offers his help . At first guards try to take him away but Leech welcomes his help . The Doctor ' s device causes the artefact to act strangely . Leech tells him the artefact was probably in the cave for thousands of years . The Doctor says it is alien , and asks if they ' ve translated it . Mel and Ly look one of the drawings , and Mel recognises a jellyfish . A background whining sound has been steadily growing louder , and suddenly they all hear plopping noises outside . It is raining jellyfish . The Doctor runs outside to investigate and sees a large spaceship , from which the jellyfish are falling . He runs back inside . Above the artefact a huge jellyfish is floating . It is a hologram . He introduces himself as Commander Hydra Sowerbii and claims the Earth for his people , the Cnidarian s . The Doctor points out that Commander Hydra ' s jellyfish are landing on dry land . The hologram disappears for a moment while he stops the jellyfish . While he is gone , the Doctor tries to work out a plan . One of the guards , Baines , reports that jellyfish are in the lake . Commander Hydra returns , and the Doctor suggests they talk . The Commander complies by sending a metal tentacle into the room to grab the Doctor . He is brought up to the ship . Baines and the other guards leave to deal with the jellyfish outside . Leech collapses , and Mel and Ly take him to the medical wing . In the Cnidarian ship , the Doctor is given an air bubble . In the medical wing , Mel sees that seven of the university students are in beds . They look awful ; suddenly Laura sits up and demands to be put in water . Mel and Ly see that she and the others are developing tentacles . Mel wonders why there are only seven students present ; eight were part of the exploration party . Ly answers that she was the eighth . On the ship , the Doctor tells the Commander that he cannot have Earth . When the Commander throws him across the room , the Doctor accidentally bites his tongue . He realises something important . . . Ly explains to Mel that the eight students had their life forces absorbed by Commander Hydra . She also has been giving Leech zooplankton in his water , changing him into one of them too . She advances on Mel , who backs away and grabs something behind her . She throws it at Ly , who screams . It was a bag of saline . Mel contacts Baines and tells him the jellyfish are vulnerable to salt . On the ship , the Doctor informs Hydra that Earth ' s seas are salt water , but the Cnidarians are fresh water . Mel then contacts the ship and tells Hydra that Ly wants to come aboard . Ly is brought aboard , where she dissolves a pack of table salt in the ship ' s water . The artefact is returned to the ship , and the Cnidarians leave Earth . The students are recovering , thanks to saline drips . The Doctor and Mel leave , with the Doctor making bad jokes . Continuity . " to be added "