Alfred Wallis and Ben Nicholson, Compton Verney, Warwickshire

It proved to be a strangely fruitful meeting of minds and artistic sensibilities. Ben Nicholson met the old Cornish fisherman Alfred Wallis in St Ives one day in 1928, quite by chance, when he was visiting with his friend and fellow painter Christopher Wood. He saw a bunch of paintings nailed – yes, nailed – to a wall beside an old fisherman's cottage. They were of boats, sea walls, sea turbulence – the compelling stuff of folk art. Wallis had come to painting late. He had started at the age of 68, after the death of his wife.

This encounter proved to be a formative one for Nicholson. That year he had been making landscapes that inclined towards naivety. The freshness of naivety, that's what he sought. No learning to stand between himself and the quick, spontaneous vision of which the eye could surely be capable. What he saw in Wallis's paintings were just those qualities that his own seemed to be seeking – an untutored freshness of approach, a fiercely compelling, if not rash, immediacy. He began to patronize the old man, and to buy his paintings for the price of a meal or two. After he returned to his smart home in London, Wallis continued to send him batches, bound up with string and brown paper. Nicholson's friends bought them too. Wallis began to be lionised a bit by the London avant garde – Herbert Read and his friends.

This exhibition at Compton Verney in Warwickshire sets a range of Wallis's paintings (mostly his seascapes, but some landscapes too) – beside Nicholson's own of the same period – from the ends of the 1920s to the beginning of the 1940s. We look from one to the other, seeing what Nicholson has learnt from the old man – how to turn a painting from a seamless, smooth, high-art thing into a rough and ready, man-made object, something between painting and sculpture. Wallis plays with perspective in thrillingly alarming ways. Old terraces of houses seem to slide into each other like rolling stock that's run out of control. We look down at some tumultuous harbour scene from a great height, as if we onlookers are gulls on the wing. It's all rather thrilling in its lack of calculation or premeditation. Fish are giants in Wallis's world, mighty leviathans, nothing more nor less than the souls of men (as he once described them), often larger than the hapless boats that hoped to snare them. Best of all is the way he paints the relationship between boats – steamships or triple masters, Wallis was old enough to have seen the shift from one to the other – and the sea itself. These ships don't half take a pummelling, pitching about, half rolling over, always at the mercy. Never has there been a sea quite so unsafe for boats as the one that we stare at in Wallis's paintings. Nicholson never goes in for any of this terror – it's not within his temperament. He learns much else though.

Let's just hope that he gave the poor old fisherman far more money than he asked for the stuff that so beautifully adorned – we see it in the photographs – his elegant London mantelpiece.

To 5 June (01926 645500)