Nemesis

Chakotay is on a survey mission on an alien planet when he is shot down. He meets a group of guerrilla warriors who are fighting their 'nemesis' and he bonds deeply with these new found friends. Meanwhile, Captain Janeway on Voyager is communicating with the Ambassador of the alien race that lives on the planet, a race that is also engaged in war with their 'nemesis'. They offer to help locate Chakotay and deliver him back to Voyager. The situation seems well in hand before we realise that something sinister lurks in this seemingly straightforward rescue mission. The concept isn't original for Star Trek, but the execution is stellar.