Blitzen fish, or Resurrection fish, were a pan-dimensional species native to the Aurora Gloriousia, in the fourth universe. They were able to mimic death whilst using a nearby power source to grow larger.
According to Gauntlet's Bestiary of the Fourth Universe, the longest known Blitzen fish was noted to have been 14.7 metres. (AUDIO: Relative Dimensions)
Whilst on the planet Quinnis, the First Doctor bought his granddaughter Susan a shrivelled up baby Blitzen fish. They both assumed it was dead but, due to its beautiful colours, Susan wanted it. (AUDIO: Quinnis)
Long after her departure, it was kept with her other possessions in her room, which the Doctor preserved untouched on the TARDIS holding ring.
One Christmas day, the Eighth Doctor recovered her room and wished to give it to Susan again as a Christmas present. Unbeknownst to the Doctor, the fish had grown up to seven metres and had hibernated in Susan's room. After reshuffling the location of Susan's room, the Blitzen fish was awakened and started breaching the planes of reality within the TARDIS, jumping back and forth in time, killing Fledershrews along the way.
The Eighth Doctor, Lucie Miller, Susan and Alex managed to release it back into its own region of space, where it couldn't harm anyone. (AUDIO: Relative Dimensions)
Behind the scenes
- The Blitzen fish is actually based on the lionfish, which bear similar appendages and stripes. The cast were shown pictures of the fish to help them to visualise what they were being attacked by.