Dyma bedwaredd nofel Manon Steffan Ros yng nghyfres yr Onnen. Enillodd Trwy'r Tonnau (dilyniant i Trwy'r Darlun) wobr Tir na nOg 2010. Mae Prism yn dilyn hynt a helynt Twm a Math sy'n dianc o'u cartref ac yn mynd i deithio o amgylch Cymru, gan aros ym Mhwllheli, Aberdaron, Porthmadog, Aberystwyth, Llangrannog a Thyddewi. Enillydd gwobr Tir na n-Og 2012.
Twm and Math are due to visit their father in his Cardiff flat during the summer holidays. Math is disabled and the father is unable to come to terms with that. The father sends his ex-wife an e-mail to eplain that he can't have the children, after all. Twm accidentally finds the message and deletes it before his mother reads it. Tir na n-Og Award winner 2012.