Answer:
The fresh muddy imprints of a pair of bare feet startled the two boys. What was a man in bare feet doing on the steps of a house in the heart of London? And where had the man gone? A remarkable sight caught their attention as they gazed. A new footprint appeared out of nowhere! Footprints continued to descend the steps and progress down the street, one after the other. The boys followed, fascinated until the muddy impressions became fainter and fainter until they vanished entirely.
The solution to the mystery was actually quite simple. The perplexed boys had been trailing a scientist who had recently discovered how to make the human body transparent. Griffin, the scientist, had conducted experiment after experiment in order to demonstrate that the human body could become invisible.
Finally, he ingested certain rare drugs, which caused his body to become as transparent as a sheet of glass while also remaining as solid as glass. Griffin, despite being a brilliant scientist, was a lawless individual. His homeowner detested him and repeatedly tried to evict him. Griffin set fire to the house in retaliation. He had to strip naked in order to flee without being seen. As a result, he became a homeless wanderer, without clothes, money, or identity — until he stepped in some mud and left footprints as he walked!