Fuck. It was a long time since he'd had anybody else in his head - properly, more than just the faint echoes the Doctor had let him feel occasionally back on the Valiant - and he wasn't prepared. He wasn't ready. Not for that sudden, blinding, overpowering presence in his mind.
He let out a faraway gasp of fright and instinctively tried to back away, both mentally and physically, knuckles whitening on the arms of the chair, mind scrambling to put up barriers he didn't have anywhere near enough control to form.