I think the core problem with this story is that, for a multi-doctor story, the multi-doctor elements just don't come into play for a while. This would work if it were a twist that the Second Doctor was involved. But as he's literally on the cover, it does partially feel like waiting around for him to turn up. Whilst it's good and does make the middle more interesting by adding a new element, it just feels underwhelming. The interactions between 3 and Jamie feel very muted, as if they were cut for time. And what we do get is just Jamie questioning the Doctor to prove it's really him - which is just not what anyone particularly cares to hear. The interactions with the Brigadier, Jamie, and 2 are good enough, though. Yet also, 3 and 2 don't interact until near the end. Whilst we have seen this pair before, it would be nice to see them interact more.

I think the idea of the aliens wanting to cull the Earth for ecological conservation of future species is really good, and I'd love to see more of these aliens in the future. It also feels very 70s in the best possible way as a plotline. The 70s vibe is something, I think the entire atmosphere of the story does really well, particularly the music.

The side characters are also very well written, with Sergeant Sinclair, Denise Walker, and Dr Jane Broadbelt being particular standouts.

However, I think the other alien species, the Greshtrenorfeel a tad pointless. Aside from kidnapping the Doctor and providing the resolution (which considering the Second Doctor was sent by the Time Lords should have been his role in my opinion) they only really seem to be there to provide a twist that, whilst it fooled me, the Doctor and Liz themselves don't fall for it and tell the audience as much. I think the main issue the story has is that it is a 7-part story with a plot that could fill a 5-part story which led to some padding.

Overall, it is good. I liked it. But I feel like it could have been a lot better if it was shorter and had more re-writes.