The Doctor and her Companions are on a junk planet searching for a crucial piece of machinery when they uncover a sonic mine. It detonates and the four awaken aboard a medic assist ship operated by a company called Tsuranga. The ship is completely automated, staffed by only two medics, Mabli and Astos.
The Doctor is intent on getting back to the T.A.R.D.I.S. and Astos has to remind her that there are other patients on board and that his job is to keep them safe until they can get to a base for treatment. The Doctor comes around and admits that the other patients should be the priority. The other patients include a Gifftan male, Yoss, about to give birth after a typical week-long pregnancy and General Eve Cicero traveling with her brother, Durkos, and android consort, Ronan.
While The Doctor and Astos are talking about how the routing has brought them into disputed territory, the ship encounters some object which manages to breach the shields and enter the ship. Sensors and power sources start to go offline.
They detect a lifeform in the ship's life-pod and split up to track it down. Astos gives the Doctor a communicator but warns her to stay off the channel he shares with Mabli as she's relatively new and doesn't need to know about the problem.
While they are tracking down the lifeform, Ronan requests more adrenaline blockers from Mabli. He threatens her with an official complaint if she does not provide them immediately.
Graham catches Durkus trying to hack his sister's medical files.
Astos finds the pod where the lifeform is hiding but gets trapped in the pod and the lifeform jettisons it with no life support. He contacts Mabli to say goodbye and that he believes in her.
The Doctor finds the lifeform eating parts of the ship. The Companions and Mabli come in and see it as well. The Doctor tries to order the lifeform off the ship but it ignores her and continues to eat the ship. When The Doctor tries to scan it with her sonic, it eats that too and then spits it out. The Doctor isn't able to turn it on after that.
Gathering in the navigation suite, Mabli is able to use the computer to read her ocular implant to identify the lifeform. It is called a Pting and it is indestructible and toxic to all organic lifeforms. It only eats inorganic matter.
The Doctor asks her team to gather everyone into the assessment area in seven minutes while she thinks of a plan. Ryan and Yaz go to let Yoss know and he shares his baby's kicking with them. He explains that he plans on giving the baby up because he's not ready to be a dad. On their way back to the group, Yaz and Ryan discuss Ryan's dad and how his mom died. Durkus comes upon Eve and Ronan administering the adrenaline blockers and is angry, knowing that she's been lying to him about her condition.
While Mabli and the Doctor are trying to imagine a solution, the Tsuranga command at Resus One alerts them that they have detected a Pting onboard. The Doctor denies it because a confirmation means command will auto-detonate the ship. Mabli warns her that they can only deny three times. After that, auto-detonation becomes the protocol.
Once the group meets, The Doctor informs them all that Astos is dead and there is a Pting aboard. She, the General, Durkus, Ronan, and Yaz head to the navigation suite to see if they can get the ship to Resus One faster by manually piloting the ship. Yoss' internal fluids break and he goes into labor and he begs Graham and Ryan to be his doulas.
On their way to the navigation suite, The Doctor is pulled aside by Mabli who hints that the General's condition may be a problem. She can't break patient confidentiality but encourages The Doctor to check her out herself.
They find the particle accelerator chamber and The Doctor charges Ronan and Yaz to guard the ship's power source from the Pting. In the navigation chamber, Durkus says that he can build a rig to manually pilot the ship if Eve can pilot it. While he's working, The Doctor talks to her about her condition and guesses that it is pilot's heart, a condition where surges of adrenaline around the heart can kill her.
The Pting breaks into the particle accelerator room and Yaz and Ronan stun it momentarily. Yaz wraps it in a mediblanket, runs it out of the chamber and then punts it down a corridor when it starts to regain movement.
With Durkus suiting his sister up for piloting the ship, The Doctor comes to the realization that the Pting is seeking out power sources. She denies the Pting detection a third time and then runs to the particle accelerator room because the bomb the Tsuranga command will use to detonate the ship must be built into the anti-matter propulsion system. She removes it and she and Yaz take it to the ship's where she sets it to detonate sooner than expected in order to attract the Pting.
The Pting enters the airlock chamber, takes the bait, and eats the bomb. The Doctor shuts the hatches of the airlock, watches the detonation do the Pting no harm at all and then jettisons the Pting out the airlock.
Eve is enjoying the sensation of piloting a ship again but her heart starts to give out from the adrenaline and Durkus has to hold her up to keep the ship flying. The siblings have a final exchange of sentiment.
When The Doctor returns to navigation, Eve is lying dead on the floor and Durkus is piloting the ship. They docked safely at Resus One where the ship and the people aboard all need to be scanned for clearance. They will provide The Doctor and the Companions with a teleporter to get back to the planet where they left the T.A.R.D.I.S.
With Eve dead, Ronan will be shut down as his service has ended. He and Durkus make peace and Durkus asks Ronan to incant Eve's final rest. The Doctor, Mabli, the Companions, and Yoss all join in the prayer.