When a man dies after falling from the top floor of a multi-level car park, the Doctor and Jo wonder why it should be of interest to UNIT. Then they see the protuberances on the man's back - As he fell, he tried to grow wings.
The Doctor and Jo land on Port Anvil - a bleak, abandoned mining colony on the remote planet Burnt Salt. A huge armored crate has recently arrived in the almost derelict Spacehub. No-one knows who it's for. No-one knows what it contains.