Trees suddenly grew over night and covered everything in the world. The Doctor and Clara spent the episode escorting the children from Clara's class around the jungle that was now in London and tried to figure out what happened.

Turned out the trees were incredibly intelligent and had detected a coming solar flare that would destroy the Earth, so they grew to cover everything and when the solar flare came the trees absored all the energy and conveniently disappeared into fairy dust after.

Oh, and one of the little girls in Clara's class was psychic and her sister had gone missing the year before, so when the little kids used the TARDIS to send a message to the governments of Earth to "please don't destroy the trees" the psychic girl also asked her sister to come home. The last scene was her and her mum returning home just in time to see one of the trees outside their house disappear and reveal the missing sister standing in pretty sparkly lights as the tree disappeared.

It wasn't a great episode.