If a plain suit lead has been trumped, it is illegal to undertrump unless by doing so one creates a K-Q or K-Q-J combination in the trick.
For the sake of clarity, it is worth pointing out that where a lead of a plain suit has been trumped by the second player to a trick and the third to play also has no cards in the suit led, then the third player must still overtrump if possible.
The second player to any lead must follow suit if able; must trump if able when void of a plain suit lead; and in any case must win the trick if he can.
The remaining cards comprise 4 plain suits, in each of which the cards rank A, T, K, Q, 9, 8, 7 (lowest).
The suit of the lowest of these cards, in plain suit order, determines trumps.