As a species, the Slithergees are fundamentally a simple race, being little more than giant, near-blind slugs. However, an interesting detail about this species is that The Doctor technically never defeated them, as his encounter with them was ‘interrupted’ due to the temporal complexities of the situation.
While the fine details of the end result varied, the Slithergees’ initial contact with humanity was fairly straightforward. Having lost their own homeworld in some unspecified accident, the Slithergees made contact with the human colony of Puxatornee in 3060, asking for one of the planet’s moons to be given to them as a colony or they would declare war on the humans. At this point, two alternate timelines were created – each one apparently based on the unwilling intervention of the Doctor – based around whether or not colony President Bailey lived beyond Christmas 3060. In the timeline where her secretary Clarence was killed because he was allegedly a Slithergee assassin and Bailey was faced with an earnest plea to find peace from people claiming to be from the future, Bailey conceded to the Slithergees’ request, while in the timeline where she was killed and discovered half-undressed with Clarence, her grieving husband Mitchell convinced himself that Clarence was a Slithergee-sent assassin and declared war (In reality, Clarence and Bailey were just having an affair and Clarence never had any connection to the Slithergee, but in the timeline where Clarence died Bailey had to accept the claims of her ‘saviours’, while in the timeline where both died Mitchell just refused to accept the idea of an affair due to his fixation with a wife who never felt the same intensity for him).
Unfortunately, regardless of whether or not Bailey lived beyond 3060, neither timeline worked out well for the humans. In the timeline where she was killed, Mitchell declared war on the Slithergees, resulting in a prolonged conflict that ended with the Slithergees nuking Puxatornee, leaving it irradiated and facing death within the next thirty years even as the Slithergee were defeated by 3070. By 3090, limited supplies meant that only some of the population were even allowed food, with ‘rationalisation’ resulting in entire families being killed to save those who were seen as more valuable to the general population, and the military using a disintegration weapon known as ‘vape guns’ that The Doctor noted weren’t good for the ozone layer. In the timeline where Bailey lived, she gave in to the Slithergees’ demands to avoid war based on the recommendation of the people who ‘saved’ her from Clarence, but her increased concessions to them led to the Slithergee basically taking over the planet. Initially just granted the first moon to live on, the Slithergee then made ever-increasing ‘requests’ for more and more concessions to be made in their favour, until, by 3090, humans were forced to act as sight guides for the ‘poor, blind, helpless’ Slithergee, who would ‘humbly’ insist that the humans execute anyone who spoke against them for hate crimes despite the subsequent retribution being excessively against the humans.
In either case, these timelines were disrupted when the Seventh Doctor and Melanie Bush arrived on Puxatornee on Christmas Day in 3090, looking for leptonite crystals to deal with a Quark threat in another part of the galaxy. In the appeasement timeline, The Doctor and Mel were taken for interrogation by Professor Capra and his ‘mind peeler’ interrogation machine, while the war timeline saw The Doctor and Mel forced to hide and search for the time machine that Capra had been working on in this reality (In the appeasement timeline Capra was portrayed as a sadistic man who enjoyed torture while the war timeline presented him as a broken genius trying to undo his planet’s hardships even after learning that his family had been ‘rationalised’ to prioritise his work). In each timeline, Stewart and Reed – lieutenants in the president’s security force in the war timeline and members of an underground dissident movement in the appeasement timeline – were able to force The Doctor to take them back in time to try and change the outcome of Bailey’s assassination (Although in the timeline where Bailey died The Doctor only went back because Mel and Stewart had already been sent back by Capra’s time machine, while in the other timeline The Doctor conceded to their threats against Mel). Back in the past, Stewart and Reed succeeded in their mission – either to kill Clarence or kill Clarence and Bailey – and then returned to 3090 with The Doctor.
However, in each case, Stewart and Reed were dissatisfied with the new future, arguing that in the war timeline the humans still had their dignity even as they faced death or that in the appeasement timeline there was always hope for future change while the war damage eliminated even that hope. The Doctor recognised the complexity of the situation, but faced with the revelation that his and Mel’s local counterparts in this new version of history would soon arrive, he decided to simply return to the TARDIS with Mel and depart, having recovered the leptonite crystals they were looking for from Capra’s machine in each reality. While he could have simply moved the TARDIS a short way in space to warn their counterparts about the situation, The Doctor concluded that things on Puxatornee were dangerous and confusing enough as it was, so it would be easier for him and Mel to get back to their original crisis and leave their ‘new’ counterparts to ‘sort things out’ (Most likely each TARDIS ‘merged’ with their counterpart once they left Puxatornee as the chain of events in each timeline was fairly similar, past experience confirming that divergent timelines can come back together after the initial split).
However, since the war timeline saw Puxatornee being destroyed when Capra’s time machine overloaded due to power supply problems, while the appeasement timeline was last seen with the Slithergee in complete control of Puxatornee after killing Mitchell as it was no longer ‘appropriate’ for a human to be President, the best-case scenario would seem to be that the appeasement timeline remained so that humans from other worlds could assist their oppressed fellows. Alternatively, it may be that the Time Lords were able to step in and amend The Doctor’s mistake, but since no reference has been made to these events since, it is left open what happened to Puxatornee and the Slithergee in the end as far as the wider universe is concerned.