What is the Japanese plain form of 行きましょうか?

Question:

Answer:

I'm not sure what you mean by "plain form"
but it's conjugated from 行く

行かない Irrealis form
*行きます Continuative　→ 行きましょう（Lot's go. Shall we go～?）
行くとき Attributive
行けば Hypothetical
行け Imperative
行こう Volitional
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