Mawdryn Undead is a fun enough romp and was my first ever Fifth Doctor story that I had stumbled upon. There's memorable aspects to this serial, like the introduction with Turlough in the car, and the Doctor getting to meet the Brigadier was great, too.

Some of the effects, dialogue, and acting, however, do leave quite a bit to be desired. It's not perfect, and the whole Black Guardian corrupting Turlough thing kind of makes the Doctor and crew seem a bit like chumps for trusting this new companion, but I do get the Doctor is just kind of stringing him along to figure things out. The inconsistency of this serial holds back what could have been great, but I still had fun overall and some of the ideas here were around regeneration and even the Black Guardian's return was all pretty compelling. Final point - Tegan and Nyssa didn't feel like they had much to do here but were pleasant enough presences.