The king cleansed his head at a tank, but a baby goldfish (śaphara-śāvaḥ) got into his skull. Physicans couldn't cure the pain, so the king prepared to die, and banished all the physicians from the kingdom, throwing their medicines into the river.
Indra told the Aśvins about this, and they went to the king's court, disguised as brahmans. They make the king unconscious with moha-cūrṇa and took his skull out and put it in a skull-shaped basin. They removed the fish and threw them into a dish. They reassembled his skull with glue, and woke him up with a reviving medicine (sañjīvanī) and showed him the fish.
Saradaprosad Vidyabhusan (ed.) भोज-प्रबन्धः श्रीबल्लाल विरचितः The Bhoj-Prabandha of Sree Ballal (With English Translation) (Calcutta: Auddy & Co., 1926), pp. 222-228.
See also the tr. by Louis Gray in the AOS series, New Haven, 1950.
The Bhojaprabandha Ballāladeva of Benares, apparently 16th century (so not toe be confused with Ballālasena the father of Lakṣmaṇasena).